1. Field of the Invention
The present invention relates to a wavelength converter, a method of manufacturing the wavelength converter, and optical devices using the wavelength converter, and more particularly, to a wavelength converter, a method of manufacturing the wavelength converter, and optical devices using the wavelength converter, which have a polymeric optical waveguide in which insertion losses of signal light and pump light are minimized and wavelength conversion characteristics are improved.
2. Description of the Related Art
In general, techniques of manufacturing optical devices include an optical communication technique, an optical device technique, an optical wavelength conversion technique, and an optical wavelength conversion technique using a second-order nonlinearity.
The optical wavelength conversion technique among these techniques of manufacturing optical devices mostly includes a difference frequency generation (DEG) technique and a cascading technique in which a sum frequency and a difference frequency are sequentially generated.
Also, the optical wavelength conversion technique using a second-order nonlinearity can be usually characterized with waveguide properties of optical wavelength converters. The characteristics of optical wavelength converters vary depending on a material used for a waveguide, in particular, a core layer, a phase matching method, and a fabrication method. Here, nonlinear optical materials, that is, oxide single crystalline such as LiNbO3 and LiTaO3, and compound semiconductor such as AlGaAs, InGaAsP, and InGaP, and nonlinear polymers are used for the waveguide of optical wavelength converters. At the present, the waveguide of optical wavelength converters is formed of a nonlinear polymeric layer having a high nonlinearity. In this case, phase matching is achieved mostly by a modal dispersion method and a quasi-phase matched (QPM) method.
It is advantageous that the above-mentioned nonlinear material has a much higher nonlinear coefficient than those of oxide single crystalline layers and compound semiconductor layers. However, quasi-phase matched (QPM) efficiencies are degraded.
More specifically, an article entitled by “Vertically stacked coupler and serially grafted waveguide: hybrid waveguide structures formed using an electro-optics polymer” to T. Watanabe in Journal of Applied physics, vol. 1, 1998, pp. 633-659 describes that when a waveguide is formed by a QPM method, normalized conversion efficiency is very low of less than 0.5%/Wcm2 in a second harmonic generation (SHG) device formed of nonlinear polymer. In addition, an article entitled by “Modal dispersion phase matching over 7 mm length in overdamped polymeric channel waveguides” to M. Jager in Applied physics Letter, vol. 12, 1996, pp. 4139-4141 describes that when a phase matching is achieved by a modal dispersion method, normalized conversion efficiency is in a relatively low range of 14%/Wcm2 in an optical device formed of nonlinear polymer.
In the case of a wavelength converter having a channel-shaped waveguide formed of nonlinear polymer, the thickness of a core layer of a waveguide of the wavelength converter is about less than 4 μm, whereas the diameter (thickness) of a core of single mode optical fiber for inputting light to the waveguide is about 8 μm. As a result, there is much difference between the thickness of the channel-shaped waveguide and the diameter of the core of single mode optical fiber. Thus, when signal light and pump light in the single mode optical fiber (having a predetermined diameter) are inputted into the channel-shaped polymeric waveguide, a very large insertion loss occurs.
Theoretically, when the pump light and the signal light are simultaneously guided with single modes in the channel-shaped polymeric waveguide, field overlap between the pump light and the signal light is most efficient, and thus effective wavelength conversion is possible. However, the thickness of nonlinear polymeric core layer for the simultaneous single-mode guiding should be thin of less than 1 μm in the channel shaped polymeric waveguide. In this case, the insertion loss is further increased leading to the decrease of conversion efficiency in polymeric wavelength converter.